The present invention relates to a subscriber line access circuit for use in a telecommunications system. The subscriber line access circuit includes a power supply bridge in the form of a first resistor inserted in the b-wire (ground potential side) of a subscriber line, a second resistor inserted in the a-wire (supply potential side) of the subscriber line, and a capacitor which bridges the two wires at the terminals of the two resistors on the sides thereof opposite to the respective potential sources. This supply bridge is coupled on the capacitor side with a two-to-four wire hybrid transformer for the two-wire/four-wire transition to the system and includes an evaluating circuit which delivers an indicating signal when the subscriber actuates a ground key.
The subscriber line access circuit also includes a cutoff circuit arranged in the power supply bridge in series with at least one of the two resistors. The cutoff circuit is provided with a cutoff switch, bridged by a high valued resistor, which is opened, after a delay, upon ground key actuation of the subscriber, and closed upon termination of the ground key actuation. Also included in the subscriber line access circuit is a device for call signal generation. With such a cutoff circuit it is possible to avoid damage to circuit elements in the remaining part of the power supply bridge due to increased current flow upon ground key actuation of the subscriber which exceeds a prescribed length of time.
It has been proposed to insert this cutoff circuit into the a-wire of the subscriber line and to bridge it with a resistor having a sufficiently high resistance value that, with the cutoff switch open, the residual current flowing through it due to ground key actuation does not damage the circuit components lying in the affected portion of the access circuit and having a sufficiently low resistance value that the current flowing through it, without ground key actuation and due to a leakage current between the wires of the subscriber line in excess of a permitted limit value, causes the evaluating circuit to respond in the same manner as with a closed cutoff switch and simultaneous ground key actuation. In this way, it is possible to test the subscriber line for insulation faults at no extra cost in equipment by utilizing for this test the evaluating circuit provided for indication of ground key actuation.
When coupling a device for generating call signals to the subscriber line access circuit, a DC voltage as well as a superimposed AC signal at the call audio frequency are applied to the subscriber line. This DC voltage and AC signal are repeatedly connected to the access circuit in accordance with the signal-to-interval ratio of the particular call. In order to prevent the current originating from the power supply from completely compensating or cancelling the current resulting from the DC voltage, it is necessary to cut off the supply current source simultaneously with the application of the DC voltage as the device for generating a call signal is switched in. For this reason, it has been the practice to provide a reversing switch for the device for generating a call signal.
An object of the present invention is to simplify the arrangement for switching the device for generating a call signal into a subscriber line access circuit of the type described above.